This invention relates to lasers and to laser gyroscopes and the techniques for suppressing off-axis resonant modes which can interfere with the proper operation of such devices.
Light waves traveling a closed optical path on a rotating platform experience an apparent shift in frequency which is proportional to the angular velocity of the platform. The sign of the frequency shift is determined by the sense of the platform rotation relative to the direction of propagation. The frequency shifts of light waves traveling in opposite directions around the same closed path will thus be the same in magnitude and opposite in sign with the result that the difference in frequency of the counter-propagating waves is a measure of the angular velocity of the platform. This phenomenon provides the basis for a ring laser gyroscope, a device for determining the rotational speed of a platform from the difference in frequency of light waves propagating in opposite directions along a closed optical path.
Counter-propagating light waves for a ring laser gyro are established in a resonant cavity which includes a number of mirrors so arranged as to cause light waves to travel in closed paths around the cavity. Such a resonant cavity generally supports a large number of resonant modes of propagation where each resonant mode is characterized by a particular frequency and a particular distribution of power in planes transverse to the propagation path. The fundamental resonant mode has a concentration of power in the central region near the axis of the transverse plane while other higher-order resonant modes have concentrations of power that extend into peripheral or off-axis regions. These off-axis modes have slightly different frequencies than the fundamental mode and their presence degrades the measurement accuracy of the ring laser gyro. For this reason, it is desirable to provide some means of suppressing these off-axis modes thereby leaving only the fundamental mode as the means for sensing platform rotation.
The means for suppressing off-axis modes resides in the distribution of resonant mode power in the planes transverse to the propagation path. The power associated with the desired fundamental mode is more concentrated in the central region of the transverse plane than is the power associated with the undesired off-axis modes. The use of some kind of aperture can therefore block the propagation of light waves in the peripheral regions of the transverse plane thereby suppressing off-axis modes without disturbing the fundamental mode.
The earliest approaches to off-axis mode suppression capitalized on this idea by incorporating a small aperture in the gyro block during the machining process or utilizing properly-chosen cavity wall dimensions and wall imperfections to obtain the spatial aperturing required to suppress the unwanted modes. These approaches have two major drawbacks: (1) the resulting apertures are not adjustable thereby preventing fine tuning of the gyro once the cavity has been assembled; and (2) scattering of the intercepted light waves occurs, causing an increase in the width of the lock-in bands, an increase in the random drift, an increase in the quantum noise and a variation of the gyro bias. These effects all substantially degrade the performance of a ring laser gyro.
Two more recent mode suppression approaches which seem to eliminate certain of the drawbacks of the above mentioned configurations are described in U.S. Pat. No. 4,627,732 and No. 4,519,708. These patents describe modifications that have been made to the high-reflectivity dielectric mirrors that are typically used in ring laser gyros to reduce the effective reflectivity of the mirrors for off-axis modes.
The modification described in U.S. Pat. No. 4,627,732 is accomplished by exposing the peripheral regions of a mirror to an electron beam and thereby modifying the index of refraction of the alternating high and low index of refraction layers that comprise a dielectric mirror. As a result of the electron beam treatment, interference occurs between the light reflected from the treated and untreated regions of the mirror and the effective reflectivity of the mirror for off-axis modes is reduced.
The modification described in U.S. Pat. No. 4,519,708 is accomplished by depositing an absorptive material on a specific off-axis area of a dielectric mirror. The thickness of the deposited material increases as a function of distance from the mirror axis to minimize scattering effects. Higher-order mode suppression is achieved by absorbing sufficient energy from waves reflected from the coated area to prevent them from lasing. Again, the effective reflectivity of the mirror for the higher-order, off-axis modes is reduced.
Neither the electron beam modification nor the absorptive deposit approach is completely satisfactory. The electron beam modification is complicated and time-consuming to manufacture. The absorptive layer approach is accompanied by undesirable scattering of some amount of incident light energy.
The absorptive layer approach is also complicated to manufacture, in that the thickness of the absorptive material must be varied linearly or quadratically as a function of distance from the axis in order to minimize the scattering effects. There is, therefore, a need for an improved method and apparatus for suppressing higher-order, off-axis modes in ring laser gyros which will overcome the aforementioned difficulties.